A system has been developed to assay DNA sequence changes which arise in cells as a result of covalent modification of the DNA by chemical carcinogens. The program involves modification of a specific DNA fragment with the active form of benzo(a)pyrene, the 7, 8-diol, 9, 10-epoxide, followed by ligation of the fragment into an appropriate plasmid vector. The ligation products are introduced into cells, replicated and the progeny DNA molecules harvested and screened for changes in the sequence of the modified fragment. This protocol is designed to measure forward mutations and avoid the selective constraints of the standard back mutation mutagenesis assays. In addition, it will permit the distinction between sequence changes which arise directly from DNA modification and those which occur from error-prone replication induced by DNA damage at distal sites. Although the amplification and assay of mutagenic events occur in bacteria, appropriate vectors can be used for the same types of experiments in human cells followed by analysis in bacteria.